I have read several of the forums on Gaggia temperature stability and preheating. I enjoyed the recent “Gaggia Baby Class Modding” here. I have a new machine and did not want to dive into the mechanics for a while because of warranty, so I read about preheat, but did not go there. I looked at the forums and found these and a few others referred to in the discussion:

Flow is about 3.67 oz/25 sec or 260 ml/min. 1/4” dia Copper tube holds, volume of cylinder, 3.14*0.32cm*0.32cm*60”*2.5cm/” is about 50ml. 9-10ml/running foot of tubing. Flow at 9 bar is 260 ml/60sec or 4.3ml/sec*25sec or about 105ml. Helps for shot 1 and maybe 2, but what is recovery for the next shot, #3.

Obviously, pull 1 has preheated water incoming and some stability with that. Interesting to see how it works with 2, 3 and 4 pulls. Could preheat effectively increase boiler size for more pulls if not bled back into the tank? Any good data with a preheat system and OPV before the preheat, off of the pump?

More to follow on using the PID to hopefully stabilize the shot temperature and speed recovery.

I've been seriously thinking of buying up one of the many cheap thermoblock machines that are always showing up on CraigsList. I think it could be interesting to pull out the thermoblock (heats the water on the fly as the water is flowing through it) and mount it between the pump and boiler. I'm not sure how big they are but it'd be fun to fit one in there.

Well ... I haven't started the project (I may never start it)... it is just a thought at the moment.

But here is how I imagine it at first:The Gaggia boiler as it is stock, my surf method is to let the boiler cycle and then I start the shot just as the heater turns off. That gets me most of the way through the shot before the boiler turns back on. So the boiler is NOT pulling any current at all during the entire shot - or at least it wouldn't pull any current if the boiler wasn't cooling down from the cold water rushing in as the shot starts.

So ... I would still time my shot to start when the boiler is up to heat and with the boiler heater off ... turn on the thermoblock (I have no thermoblock experience yet so I am guessing here - I'm guessing it takes 3-4 seconds to get up to heat) and then start the shot.

The thermoblock heating the water coming into the boiler "ought to" get you all the way through the shot without the boiler wanting to cycle back on.

All this (in my mind) is without a PID at all. I can get pretty amazing temp stability with the stock boiler and no PID just by using a thermometer on the boiler and manually cycling the power to the boiler a few brief pulses as needed. (one of these days I'm going to borrow a good video camera and post a YouTube of me pulling a shot without the temp dropping and no PID)

If that prototype worked out (manually starting the thermoblock with a toggle) - and depending on what I learn by testing it (does it start heating instantly or how many seconds before the water is hot?) I could devise a circuit where you hit the brew switch and it starts the thermoblock and waits the determined time before starting the pump (which might turn out to be the amount of time I like to preinfuse anyway - so the thermoblock could be heating up as the low flow preinfuse is soaking the puck).

So you don't have a PID, you are the PID, flicking power on and off. I am the PID for steam that way. I watch the temperature and flick steam off a few seconds and then back on. That way it never shuts down the bimetal and stays off for any length of time.

You would actually use the unresponsiveness of the bimetal in you favor, allowing a large cool down before it turns the boiler back on. You could always devise your toggle to keep the boiler out of the circuit when the thermoblock is in, or perhaps you meant that.

I figured that I wouldn't need to cut the boiler out of the circuit because the boiler wouldn't cool down and wouldn't trigger the thermoswitch. But, yeah, it would also be easy to wire the toggle so that when the thermoblock was on the boiler heater couldn't come on.

I have had two gaggias plugged into the same circuit before and it doesn't (usually) trip the breaker. I haven't ever even seen a thermoblock in person though - I was just thinking about it - so for all I know it is too big or just impractical for what I was thinking - and I don't know how much current they draw. I've got a spare Gaggia boiler and if there were room, I'd just add a second boiler without the bottom half (just a blank plate to seal).

Whenever I've seen a YouTube video of someone with a Gaggia (or other) with a PID, it looks great until they start the shot and then you see the temp plummet for several seconds before the PID tries to catch up.

When I am acting as the PID (as you described it), I know that I am about to start the shot so I can hit the heat for 4-5 seconds before I start the pump and that way I can start a shot where the temp doesn't drop at all. But you have to get used to the idea that the heat you turn on now won't show up for about 5 seconds - so you learn to anticipate and not wait to see the change on the thermometer. So in this way, I think my method is better than a PID (unless someone has a PID that starts the heater and delays the pump to anticipate the shot) - but I'm sure a PID is very nice for holding the start temp right.

I am aware that a PID helps lock in a beginning temperature, but does not directly affect intrashot temperature in its normal usage. Some have said that it takes about 4-5 seconds for the sensor to react to the incoming cool water. I used the alarm function of the PID to turn on the heater with the pump. It cycles on and off as needed, but does heat at the same time water is incoming. Using high and low alarm settings, the heater is cycled, full on and off, not like the PID blips. I used the schematic by Jerry Skene found here

and switched from steam function to brew switch to turn on the heater with the pump. I used an old 6v cell phone charger instead of 12v. I had it lying around. The Auber generic PID worked well and the RTD sensor by Auber. Diodes are less than $1.

If I understand the formulas correctly, then 1425 watts * 25 seconds for a pull is about 35kj. If a double pull is about 70 ml, then to heat 70 ml from 25c to 100c is about 22kj. If I could get the heater on for about 60% of the time then that would supply the approximate amount to heat the incoming water. I am currently trying this. The return to baseline temperature is about the same time it takes me to get the next double ready, quicker than before. I am still playing with settings, but I am coming down on the stable temperature of 218f from above instead of cooler and back up to temperature after a shot. I do not get steam with the pull but do not have the equipment to measure puck temperature as some have done.

Best of both worlds for Gaggia SBDU may be passive preheat, with OPV near the pump and temperature control during brew.

I'm always impressed when people know the math .... something that I have always struggled with.

There is no good reason that you couldn't design a PID that uses your calculations and perhaps even a flow meter to detect the speed of the cool water entering the boiler to start the heater in anticipation of the water that will start to flow and keep the shot temp steady without a preheater.

I know the heater is capable of holding the shot steady (because I can do it manually with lots of practice and trial and error). So if the PID could remember a routine or let you manually program a routine - once you figure out how the heat cycles needed for a standard double (and for a ristretto and a few others) you could pull up the temp profile you want ... hmmmm.

When I use a Gaggia without a 3-way valve, I can simply use the steam switch ... but with a 3-way valve I add a switch that lets me kick the heat on without activating the solenoid valve. My routine for a double is to get the boiler temp idling steady at my target (and make sure the boiler is full) - then I hit the heater for 4 seconds then off and start the shot - then throughout the shot it is on-off-on-off at a rate of about on for 2 seconds off for 2 seconds. When I get it right, the temp doesn't vary at all (or maybe one degree) through the entire shot. Or I can let it drop a little if I want that profile. Anyway .. that's about my routine if you know a way to get your PID to emulate it. (but my idea of a double is 30 ml ristretto or maybe 40-50 occasionally - never 75 but it's all personal taste - if you were pulling 75ml shots you would need a lot more energy than I use to keep the temp steady)

I had thought about something as simple as a variable oscillator (I'm a musician, I have a metronome that is easy to speed up and slow down) to cycle the power. Or with a moog synth I can make a square wave at any frequency I want and I can vary how wide the pulses are - so something similar could control the heater cycles (I'm so clueless about PID - it probably already does all that if you know how to tell it to do it).

Another thought I had ... the Gaggia has TWO heaters and for 110-120volts they are just both wired parallel (for 220 I believe they are in series) so for 110volts you could simply turn on one heater and not the other when you start your shot and half power would probably be the perfect amount to keep the temp up during a double (since my manual routine has me cycling the power on and off about equal amounts). that could be as simple as a single toggle switch to start half power just before you start the shot.

Interestsing that you cycle on and off about 2 seconds, or heat about 50% of the time. My math guesstimate is about 60%. The heater can overpower the incoming water, and you need about 50 - 60% power time. I could probably start power with a preinfuse, but I am not convinced that it is necessary, either preinfuse with Gaggia, or earlier power. I had a Delonghi 701 and had never seen a forum until about 6 weeks ago. I had learned to use the steam switch, temperature surf, before I knew a name for it. The Delonghi was old and not heating as well, and the steam switched helped.

If I understand the flow into and out of the boiler, incoming is low and outgoing is through an upright tube. The incoming water is not outflowing until there is mix, or layering, depending on turbulence/flow rate. I am not having pjroblem cooling, that I know of, turning on heat with the pump. Recovery is done quickly and as noted can be from above temperature, not below. What may be needed, to evaluate the system beyond my observations, is for someone with the equipment to measure puck temperature.

I am new at this. As noted I had a Delonghi for years. I got the Gaggia at the end of July, and the PID about 3 weeks later, and I did not even know the term PID prior. Only very lately started playing with the boiler/pump combo. I think it has potential. So a question Andy, are you anti PID? You seem to do the function of one. I think that it is best to learn manually and understand the machine function, but you are literally being the PID, yet seem against it.

I'm not at all "anti-PID" ... I simply don't have one and my only experience is seeing them in action in YouTube videos and seeing the temp drop when the shot starts.

I'm not familiar with the Delonghi - but I have a Briel with a small stainless steel boiler with the heater on the inside, submersed in the water. The temp routine on that one is very different than on the Gaggia.

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